The ImportError raised on an import failure with the uno module loaded
now includes a complete traceback and the original Python exception
message text, combined with the most relevant (nearest to failure if
imports are nested) uno lookup that also failed.

Change-Id: Id968d84d7f09d555a81017a99369beb503d61439
Signed-off-by: Stephan Bergmann <sbergman@redhat.com>